Fluorinated polymers are well known for their physical properties such as corrosion resistance, low coefficients of friction, chemical resistance, and thermal stability. An example of such a fluoropolymer is polytetrafluoroethylene (PTFE), which is widely used in medical and industrial applications. PTFE has an extremely high melt viscosity, which makes processing by melt fabrication or injection molding very difficult. Full density PTFE is also generally non-elastic or semi-rigid, making it unsuitable for applications requiring some degree of elasticity.
Elastomeric fluoropolymers, such as KALREZ® elastomer available from E. I. duPont de Nemours & Co., combine the advantages of a fluoropolymer with the mechanical aspects of an elastomer. The mechanical and elastomeric properties of such materials are, however, a result of the cross-linking or curing, which improves strength and recovery, and renders the polymer a thermoset. The incorporation of a cross-linking system, which improves elastomeric properties of the thermoset, requires additional monomers to be added during polymer synthesis, and cross-linking agents to be added during milling and compounding steps. The compound is subsequently molded and heat treated to form the final article. This heat treatment is also known as curing or vulcanization. Biocompatibility of the final product is compromised by toxic agents and additives necessary for cross-linking.
In U.S. Pat. No. 3,132,123 to Harris et al., assigned to E. I. Du Pont de Nemours & Co. Inc., a semi-crystalline copolymer of tetrafluoroethylene (TFE) and perfluoromethyl vinyl ether (PMVE) is disclosed. This copolymer of TFE and PMVE was prepared in an attempt to develop a plastic material having the desirable physical properties of PTFE without the undesirable high melt viscosity, which makes melt processing of the polymer very difficult. Thus, these efforts focused on the development of a melt-processable, injection moldable PTFE substitute. The copolymer disclosed had low levels of PMVE (approximately 11% by weight), which is sufficient to yield a melt viscosity lower than PTFE but not sufficient to make the polymer amorphous. This, in turn, would enhance the ease of fabrication by injection molding. The biocompatibility of this copolymer was not addressed nor suggested.
Subsequent development efforts relating to the copolymer of TFE and PMVE focused on cross-linking systems, for the purpose of enhancing the mechanical properties and imparting elastomeric properties to the polymer. In publications by Du Pont, Kalb et al., “A New Engineering Material for Advanced Design Concepts,” Applied Polymer Symposium No. 22, 127–142 (1973), and Barney, et al., “A High-Performance Fluorocarbon Elastomer,” Journal of Polymer Science, Part A-1, Vol. 8, 1091–1098 (1970), the need as well as the difficulty of cross-linking the TFE and PMVE polymer is disclosed. These publications focus on the search for a third monomer, to be copolymerized with the TFE and PMVE, that would provide a site for vulcanization or chemical cross-linking. Barney, et al., discloses a tensile strength of 675 psi for the uncured, noncross-linked terpolymers in the form of a gum. These and similar development efforts led to the commercialization of KALREZ® elastomer, which is a thermoset, cross-linked terpolymer, containing monomers of TFE, PMVE and additional other monomers. Similar thermoset, cross-linked, terpolymers have been developed and are commercially available under the trade names CHEMRAZ® and DAI-EL PERFLUOR®. Although possessing good mechanical properties, these cross-linked systems are not suitable for medical applications, particularly implantable medical devices, due to the known toxicity of the additives.
All known copolymers containing TFE and PMVE comonomers commercially available in final form (e.g., O-rings, sealants and gaskets) are cross-linked materials containing additives and fillers. Noncross-linked resins or articles of these copolymers (generally terpolymers) are typically in the form of gums. These gums generally have low molecular weight and poor mechanical properties. They are not useful until cross-linked. The lack of commercially available resins of amorphous thermoplastic copolymers of TFE and PMVE has hindered thorough biocompatibility testing of this copolymer. To the knowledge of the present inventors, there have been no publications citing efforts to test or determine the biocompatibility of a noncross-linkable, thermoplastic copolymer of TFE and PMVE. There have only been generalized references relating to the biocompatibility of plastics made from copolymers of TFE and a perfluoroalkyl-vinyl-ether, or a perfluoroalkoxy-vinyl-ether (see, e.g., Homsy, “Biocompatibility of Perfluorinated Polymers and Composites of these Polymers,” Biocompatibility of Clinical Implant Materials, Volume II, Chapter 3, pp. 59–77 (1981)).
Presently there is dearth of suitable implantable elastomeric materials. Currently available elastomers in this field are predominantly silicones and polyurethanes, which have well documented deficiencies related to long term stability in-vivo and mechanical weakness. These deficiencies may include adverse foreign body reactions, biological response to leachable species, particulation concerns, long-term embrittlement and stress cracking. In addition, these polymers are thermosetting elastomers, which have known limitations in processability. Disclosed in U.S. Pat. No. 4,816,339 to Tu et al. are articles of expanded PTFE (ePTFE) combined with thermoset elastomers. Various thermoset elastomers are disclosed including copolymers of TFE and PMVE in cross-linked or cured forms. These elastomers are believed to suffer from similar deficiencies as those outlined above.
Thus it would be desirable to provide a perfluoropolymer in a pure, noncross-linkable, thermoplastic, amorphous form. Such a perfluoropolymer could ideally have elastic properties, high tensile strength, high purity, excellent clarity and abrasion resistance along with ease of processing.